System-Level Power Optimization for a D/A Converter for Hearing-Aid Application

This paper deals with a system-level optimization of a back-end of audio signal processing chain for hearing-aids, including a sigma-delta modulator digital-to-analog converter (DAC) and a Class D power amplifier. Compared to other stateof-the-art designs dealing with sigma-delta modulator design for audio applications we take the maximum gain of the modulator noise transfer function (NTF) as a design parameter. By increasing the maximum NTF gain the cutoff frequency of modulator loop filter is increased which lowers the in-band quantization noise but also lowers the maximum stable amplitude (MSA). This work presents an optimal compromise between these. Increased maximum NTF gain combined with a multi bit quantizer in the modulator allows lower oversampling ratio (OSR) and results in considerable power savings while the audio quality is kept unchanged. The proposed optimization impacts the entire hearing-aid audio back-end system resulting in less hardware and power consumption in the interpolation filter, in the sigma-delta modulator and reduced switching rate of the Class D output stage. Keywords—Sigma-Delta modulator; Interpolation filter; Class D; Hearing aid; low voltage, low power